As semiconductor devices are increasingly sophisticated in recent years, extremely fine patterns are formed thereon. However, it is difficult to directly form such extremely fine patterns on a photomask. A solution for this is a projection aligner that includes a photomask having a pattern larger than the actual pattern to be formed on a semiconductor device and projects the pattern of the photomask onto a wafer at a reduced scaling via a projector lens located between the photomask and the wafer.
Although being advantageous in forming fine patterns over the contact aligner that exposes a wafer with a photomask being in direct contact with the wafer, the projection aligner requires an alignment technology, or to be more specific, a technology for optimally adjusting the relative positions of the pattern already formed on the wafer and of a pattern to be formed thereon in a subsequent process.
The alignment process performed by conventional projection aligners is roughly divided into the following three steps.
Step (1): A prealignment section of a wafer loader performs a prealignment process in which the position of the wafer is adjusted by, for example, orientation flat.
Step (2): After the prealignment step, the wafer is loaded on the wafer stage, the focus is automatically set, and then the x- and y-coordinates of the wafer are determined on the wafer stage, using a global alignment mark.
Step (3): A fine alignment process is performed for step and repeat exposure.
In Step (3), Substeps (a) to (f) are executed as below to gauge correction values.
Substep (a): The wafer is measured for a scaling correction value, that is, an enlargement or reduction amount of an area, on the wafer, in which the pattern is already formed. The area will be herein after referred to as a bed shot.
Substep (b): The wafer is measured for an orthogonality correction value, that is, horizontal and vertical displacements of the wafer from the orientation flat.
Substep (c): The wafer is measured for a rotation correction value (rotation value).
Substep (d) : The wafer is measured for offsets in the x and y directions, that is, displacements of the wafer from the bed shot in the x and y directions.
Substep (e): The shot is measured for a scaling correction value. In other words, a pattern area (shot) to be newly formed on the wafer is measured for an enlargement or reduction amount.
Substep (f): The shot is measured for a rotation correction value, that is, for a rotation value of the shot to the bed shot.
The exposure position of the wafer is determined according to the correction values obtained from Substeps (a) to (f). Then alignment is done by means of the scaling correction (see FIG. 16), rotation correction (see FIG. 17), and offset correction.
Nevertheless, since the projection aligner includes a projector lens through which the pattern formed on the reticle is projected onto a wafer, shot distortions due to distortions of the projector lens, on top of the displacements (scaling, rotation, and offset), are inevitable. In other words, the shot projected onto the wafer is distorted by aberration and other deformation of the projector lens. The shot distortion cannot be corrected for by the aforementioned conventional alignment method.
The lens distortion is unique to the projector lens used. If a single projection aligner projects both the bed shot and a shot to be newly formed thereon (hereinafter, will be referred to as a current stage shot), the lens distortion does not vary and therefore results in the same shot distortion, causing no problem. On the other hand, if a different projection aligner is used for each of a series of exposure steps to achieve a high throughput, for example, in mass production of semiconductor devices, adverse effects of the shot distortion become so eminent as shown in FIGS. 16 and 17 that the alignment displacement due to the shot distortion needs be reduced.
Conventionally, if more than one projection aligners are used, each projector lens incorporated in the projection aligners is measured for its distortion in advance, and projection aligners with similar lens distortions were grouped together for actual use (grouping).
Here, such grouping of the projection aligners to reduce the effects of the shot distortion caused by the lens distortion limits the number of projection aligners usable for manufacturing semiconductor devices in large quantities and thus presents an obstacle in achieving a high throughput in manufacture.